1. Field of the Invention
The present invention relates to an apparatus for detecting an angle of rotation and more particularly to an apparatus for detecting an angle of rotation employing magnetic sensors of a ferromagnetic material to detect an angle of rotation in a digital manner.
2. Description of the Prior Art
There is an apparatus for detecting an angle of rotation of a rotary member employing magnetic sensors of a ferromagnetic material. The ferromagnetic magnetic sensor utilizes magnetoresistance effect of a ferromagnetic metal, namely, the property of the material such that its resistance value changes according to the angle formed by the direction of magnetizing force and that of electric current.
The resistance value of a ferromagnetic material becomes the maximum when the direction of the magnetizing force and that of the electric current are in parallel and becomes the minimum when they cross at right angles. Therefore, when a ferromagnetic magnetic sensor is constructed, as indicated in FIG. 5, of thin film patterns A, B of a ferromagnetic material arranged to cross at right angles and electrically connected in series and connected to a power source voltage V.sub.cc at both ends a, c thereof and then the thin film patterns are applied with a magnetic field which is sufficiently high to saturate the magnetization in the film patterns, and if the magnetic field is rotated through the angle .theta. with reference to the direction of the current through the thin film pattern A, then an output signal V.sub.0 given by the formula, EQU V.sub.0 =V.sub.cc .multidot.K.multidot.cos 2.theta.+1/2.multidot.V.sub.cc,
will be delivered from the output terminal b as indicated in FIG. 6. Incidentally, FIG. 7 shows an example of the thin film patterns A, B.
FIG. 8 indicates an example of a prior art apparatus for detecting an angle of rotation employing a magnetic sensor of a ferromagnetic material arranged such that a ferromagnetic magnetic sensor DME is disposed to face the periphery, where multiple magnetic poles are provided, of a ring magnet RMG. In this case, when the distance between neighboring poles of the ring magnet RMG is represented by .lambda. as indicated in FIG. 9, thin film patterns A, B of the ferromagnetic magnetic sensor DME are disposed at the spacing of .lambda./2.
Now, if a power supply voltage V.sub.cc is applied between the terminals a, c (refer to FIG. 9) and a voltage V.sub.0 is taken out from the terminal b, then a signal having the same number of sine waves as the number of the poles is generated for one rotation of the ring magnet RMG. Therefore, when the number of poles is represented by P, the relationship between number of pulses (sine waves in the signal) n and the angle of rotation .theta. is given by EQU .theta.=(360/P).multidot.n
and it becomes possible to detect the angle of rotation by counting the number of the pulses.
In the prior art apparatus for detecting an angle of rotation employing the ferromagnetic magnetic sensor, however, the number of poles had to be increased in order to improve its resolving power, and the diameter of the rotary magnet had to be enlarged in order to increase the number of poles, and the apparatus had to be made n a large form. In other words, there was a problem in the prior art that it was impossible to provide an apparatus for detecting an angle of rotation with a high resolving power in a small form.